Currently, natural rubber (one example of polyisoprenoids) for use in industrial rubber products is produced by growing rubber-producing plants, such as Hevea brasiliensis of the family Euphorbiaceae or Ficus elastica of the family Moraceae, biosynthesizing natural rubber in the laticifer cells of the plants, and extracting the natural rubber from the plants by hand.
At present, Hevea brasiliensis is practically the only one source of natural rubber for industrial use. It is also widely used in large quantities in various applications as a main raw material of rubber products. Unfortunately, Hevea brasiliensis is a plant that grows only in limited areas such as in Southeast Asia and South America. Moreover, Hevea brasiliensis requires about seven years from the time it is planted until it matures enough to allow rubber extraction. Furthermore, the seasons for extracting rubber from it are limited in some cases. Additionally, the period during which natural rubber can be extracted from the mature trees is limited to 20 to 30 years.
Although more natural rubber is expected to be needed, in particular, by developing countries in years to come, for the reason mentioned above it is difficult to greatly increase the production of natural rubber using Hevea brasiliensis. Depletion of natural rubber sources is therefore of concern and there are needs for stable natural rubber sources other than mature Hevea brasiliensis and for improvement in productivity of natural rubber from Hevea brasiliensis. 
Under such circumstances, natural rubber sources other than Hevea brasiliensis are being actively sought. At least 2000 types of isoprenoid-producing plants other than Hevea brasiliensis have been known. In particular, Guayule and Taraxacum koksaghyz are being studied as novel natural rubber sources. Some plants of the family Asteraceae are also known to produce isoprenoids. One example is Sonchus oleraceus which naturally grows in a wide range of areas in Japan.
If these plants are used as novel natural rubber sources, mass reproduction of these plants will be required for mass production of natural rubber. Plants may be reproduced in large quantities, for example, by a method of growing plants from seeds or a method of reproducing plants from cuttings. These methods, however, can easily be affected by weather, seasons or other factors and thus may fail to stably reproduce plants.
There are also attempts to increase the production of natural rubber using Hevea brasiliensis. Hevea brasiliensis plantlets can be reproduced by sowing and growing seeds into seedlings, raising the seedlings to prepare rootstocks, and grafting buds formed on clean plantlets to the rootstocks. The number of buds obtainable from plant clones is limited. Thus, to widely introduce superior varieties, these superior varieties of clean plantlets need to be reproduced in large quantities.
Moreover, grafting, which is a conventional clean reproduction technique, may, at the same time, allow inheritance of diseases derived from the original trees and thus may result in reproduction of plantlets suffering from the diseases. Accordingly, there is a need for methods of stably reproducing plants.
Meanwhile, a possible approach to increase the production of isoprenoids in plants is to modify plants so as, for example, to enhance stress resistance or increase the amount of isoprenoids accumulated in the plant. Plants may also be modified using artificial crossing or by mutation; however, such methods have difficulty in efficiently providing the plants with desired characteristics and thus have low feasibility. Therefore, it is considered that plants should be modified using cell technologies in which a target gene is introduced into plant cells to provide a desired characteristic.
If cell technologies are used, plant cells into which a target gene is introduced need to be redifferentiated into plants. In other words, the plant cells (e.g. calli) need to be regenerated into plants. Unfortunately, although various tissue culture studies have been made on plants, there are few studies suggesting how to regenerate plants from calli of isoprenoid-producing plants. Thus, it has been difficult to stably regenerate plants from calli.